A linear motor actuator, which is greatly used in various tables of machine tools, moving parts of industrial robots, various conveyer apparatus and the like, has functions of giving linear motion to a movable body such as a table and stopping the movable body at a predetermined position. This linear motor actuator is driven by using an AC linear synchronous motor. The AC linear synchronous motor is structured to have a stator and a mover, and an exciting coil of copper wire is formed on a part of the stator or the mover. When exciting current is made to pass through the exciting coil to generate a driving force, linear movement is achieved.
Here, the rotational speed of the AC motor (linear movement speed of the linear motor) is fundamentally proportional to a supplied frequency. Hence, the AC motor is controlled by a method of once converting current from a commercial power source into direct current and then converting the direct current as the power source into alternate current having any frequency. In the AC motor control using an inverter as a power converter for converting DC power to AC power, the PWM (Pulse Width Modulation) system is often used in which the pulse width ratio is changed by controlling the width of output voltage wave or chopping a half cycle of the output voltage wave.
In the AC motor control of controlling the number of pulses, the pulse interval, the pulse width and the off time to output a voltage equivalent corresponding to output frequency, the time constant of the motor is an important parameter. However, as the mover of the conventional linear motor actuator has a weight of a certain level or more, the influence of the electric parameter of the AC motor on the actuator is very small as compared with the influence of the mechanical time constant (inertia) and insignificant.
Associated with downsizing and weight reduction of the liner motor actuator, there has been produced an ultra-compact AC linear synchronous motor having only a few turns of exciting coil, which makes apparent the problems that 1) when the PWM system is used as-is in the ultra-compact AC linear synchronous motor, the number of turns of the exciting coil is very small and the time constant (L/R) becomes very small, resulting in quick current response to the voltage and uncontrollability of the current, and 2) although the waveform of current passing through the exciting coil is preferably a sine wave, harmonic components are convoluted to vibrate the motor or cause noise, and the like.
The following patent document 1 discloses a related art of using the inductance of the exciting coil as a motor control parameter in the AC motor control. The patent document 1 relates to a motor and its control method. According to the patent document 1, there was a problem that although the inductance of the motor coil varies depending on the number of rotation times of the motor, the temperature and the like, the motor applied voltage and motor current were calculated with use of a fixed value of the inductance, resulting in reduction in the motor efficiency. As measures to solve this problem, the patent document 1 discloses extracting a motor constant based on a use environment of the motor and generating a control signal based on the extracted motor constant. However, the technique of the patent document 1 does not serve to solve the problems of a ultra-compact AC motor control having an extremely small time constant, which differs from the present invention, and there is hardly any other document which discloses the technique to solve such problems.    [Patent Document 1] Japanese Patent Laid-open Publication No. 2002-369598